Incorporeal

From NWNWiki

An incorporeal creature is one that has no physical body. In terms of game effects, this means 50% concealment and the ability to walk through other creatures. In addition, some spells exempt incorporeal creatures from their effects.

A creature is flagged as incorporeal by setting an integer local variable named X2_L_IS_INCORPOREAL to 1 on the creature. This must be done before the creature's OnSpawn event fires (typically by setting it on the blueprint) or else the creature will not receive the concealment and the ability to walk through other creatures. Furthermore, since these benefits are implemented as extraordinary effects, they are lost upon resting.